How can I make a mirror which reflects only chosen things?
Composite view layers
You can take advantage of view layers and compositing. It's very flexible and collection-driven.
Create a collection with the objects that you don't want to appear in the mirror (s_vampire in this case).
Create two view layer and in one Disable the collection you created before: one with everything enabled called for example ref

And one othe called for example n vis with the collection disabled.

Now, just use cryptomatte layer to pick the mirror plane as a mask in the compositor:

The scene will be rendered 2 times and the mirror will show only the scene where the collection with s_vampire object is disabled. This is the result.

You can download the file here to explore the scene: